An electrically conductive coating material or an electrically conductive adhesive is in the form of, for example, an electrically conductive paste product. Depending on the use, the electrically conductive paste product can be classified into an electrically conductive paste in the narrow sense and a resistor paste. The present invention relates to electrically conductive paste in the general meaning including a resistor paste.
An electrically conductive paste predominantly contains an electrically conductive material, an auxiliary agent, a resin serving as a binder or a matrix material and a solvent, and is produced by dispersing fine particles of the electrically conductive material and the auxiliary agent into a varnish obtained by dissolving the resin in the solvent. Dispersion of the electrically conductive material and the auxiliary agent is performed by use of a dispersion mixing machine, a pulverizer or a mill such as a three roll mill, a ball mill, a paint shaker or a planetary mill. Depending on the temperature required for thermal treatment after application, the electrically conductive paste is classified into a paste which is cured through drying (hereinafter may be referred to as a “drying-cured paste”) and a paste which is cured through baking (hereinafter may be referred to as a “baking-cured paste”).
A drying-cured paste is thermally treated at a temperature falling within a range of ambient temperature to about 250° C., to thereby form a composite product containing the electrically conductive material, the auxiliary agent and the resin component. When the resin to be employed in the drying-cured paste is appropriately chosen, characteristics such as solvent resistance, thermal resistance, adhesion and flexibility can be imparted to the paste. Typical examples of the resin employed in this type of paste include phenolic resin, epoxy resin, polyester resin, silicone resin, acrylic resin and polypropylene resin. When a baking-cured paste is thermally treated at a temperature of about 400° C. to about 1,300° C., the organic component of the applied paste is burned out and the inorganic component remains in the thus-treated paste. The resin employed in the baking-cured paste is chosen in consideration of behavior of the resin during the course of application or thermal treatment of the paste. Examples of the resin employed in this type of paste include cellulose resins such as nitrocellulose and ethyl cellulose, acrylic resin and butyral resin.
In order to improve fluidity of the paste during the course of application and to improve the strength and tribological characteristics of a coating film formed through application of the paste, a dispersant or a thickener is employed as the auxiliary agent. Examples of the auxiliary agent employed include silicon oxide and alumina.
The solvent must be chosen in consideration of the solubility of the resin in the solvent, the fluidity of the solvent required for dispersing the electrically conductive material therein, the volatility of the solvent during the course of thermal treatment of the paste performed after application, and characteristics of a coating film formed after volatilization of the solvent. Examples of the solvent employed include methyl ethyl ketone, N-methylpyrrolidone, terpene compounds such as terpineol, glycol ethers and glycol esters.
Application of such an electrically conductive paste is performed by means of screen printing, the dispenser method, dipping, the transfer method, the applicator method, brush application or spraying. When the paste is applied to a substrate or an electronic element, generally, screen printing, dipping or the transfer method is employed. The viscosity, etc. of the electrically conductive paste must be regulated depending on the application method to be employed.
As the electrically conductive material, there are employed fine particles of a noble metal or a noble metal alloy such as gold, platinum, palladium, silver, a silver-platinum alloy or a silver-palladium alloy, or fine particles of a base metal such as copper, nickel, aluminum or tungsten. Alternatively, there are employed fine particles of an electrically conductive non-metallic material such as carbon, graphite, carbon black, ruthenium oxide, tin oxide or tantalum oxide.
However, the electrically conductive paste containing metal serving as the electrically conductive material involves the following problems: electrical conductivity is lowered with passage of time through, for example, oxidation or corrosion of the metal; an electrically conductive coating film formed of the conductive paste is exfoliated from an electric circuit board because of deformation of the circuit board; and the conductive paste is expensive when the metal is silver. Meanwhile, the carbon-containing electrically conductive material involves problems in that, for example, its electrical conductivity is insufficient, although the conductive material exhibits resistance against, for example, oxidation and corrosion, and is advantageous from the economical viewpoint.
Recently, there have been proposed an electrically conductive coating material containing vapor grown carbon fiber, carbon black and a thermoplastic resin and/or a thermosetting resin (see, for example, Japanese Patent Application Laid-Open (kokai) No. 6-122785); an electrically conductive coating material containing boron-containing fine carbon fiber, and a thermoplastic resin or a thermosetting resin (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2001-200211); and a coating material or an adhesive containing carbon fiber which is readily graphitized, (see, for example, Japanese Patent Application Laid-Open (kokai) No. 61-218669).
In general, an electrically conductive coating material or an electrically conductive adhesive incorporates, as an electrically conductive material, fine particles of a noble metal such as silver, gold or platinum, fine particles of a base metal such as copper or nickel, or fine particles of carbon. In contrast, the three patent publications mentioned above propose a technique in which an electrically conductive composition containing a resin component and carbon fiber is employed, to thereby provide an electrically conductive coating material or an electrically conductive adhesive, which exhibits improved characteristics (e.g., electrical conductivity and durability).
However, in the technique proposed in any of the above publications, a large amount of carbon fiber must be added to the resin component in order to obtain sufficient electrical conductivity. As a result, a mixture of the carbon fiber and the resin component exhibits lowered fluidity. Meanwhile, when only graphitized carbon fiber is employed as an electrically conductive component, the resultant electrically conductive composition incurs resistance anisotropy.